How does LINQ defer execution when in a using statement

Question

Imagine I have the following:

private IEnumerable MyFunc(parameter a)
{
   using(MyDataContext dc = new MyDataContext)
   {
      return dc.tablename.Select(row          


        

Answer 1

I would expect that to simply not work; the Select is deferred, so no data has been consumed at this point. However, since you have disposed the data-context (before leaving MyFunc), it will never be able to get data. A better option is to pass the data-context into the method, so that the consumer can choose the lifetime. Also, I would recommend returning IQueryable<T> so that the consumer can "compose" the result (i.e. add OrderBy / Skip / Take / Where etc, and have it impact the final query):

// this could also be an instance method on the data-context
internal static IQueryable<SomeType> MyFunc(
    this MyDataContext dc, parameter a)
{
   return dc.tablename.Where(row => row.parameter == a);
}

private void UsingFunc()
{
    using(MyDataContext dc = new MyDataContext()) {
       var result = dc.MyFunc(new a());

       foreach(var row in result)
       {
           //Do something
       }
    }
}

---

Update: if you (comments) don't want to defer execution (i.e. you don't want the caller dealing with the data-context), then you need to evaluate the results. You can do this by calling .ToList() or .ToArray() on the result to buffer the values.

private IEnumerable<SomeType> MyFunc(parameter a)
{
   using(MyDataContext dc = new MyDataContext)
   {
      // or ToList() etc
      return dc.tablename.Where(row => row.parameter == a).ToArray();
   }
}

If you want to keep it deferred in this case, then you need to use an "iterator block":

private IEnumerable<SomeType> MyFunc(parameter a)
{
   using(MyDataContext dc = new MyDataContext)
   {
      foreach(SomeType row in dc
          .tablename.Where(row => row.parameter == a))
      {
        yield return row;
      }
   }
}

This is now deferred without passing the data-context around.

Answer 2

I just posted another deferred-execution solution to this problem here, including this sample code:

IQueryable<MyType> MyFunc(string myValue)
{
    return from dc in new MyDataContext().Use()
           from row in dc.MyTable
           where row.MyField == myValue
           select row;
}

void UsingFunc()
{
    var result = MyFunc("MyValue").OrderBy(row => row.SortOrder);
    foreach(var row in result)
    {
        //Do something
    }
}

The Use() extension method essentially acts like a deferred using block.